Polymer Electrolyte membrane with coating layer of anion binding agent and fuel cell using same

ABSTRACT

The present invention relates to a multi-layered polymer electrolyte membrane for a fuel cell, which is prepared by introducing an anion binding substance as a coating layer to a non-aqueous polymer electrolyte membrane for preventing the elution of acid, and a fuel cell comprising the membrane. In particular, the present invention discloses a multi-layered polymer electrolyte membrane prepared by coating an anion binding substance on a non-aqueous polymer electrolyte membrane, and a fuel cell comprising the membrane, thereby preventing the elution of acid and maintaining the performance of a fuel cell to economic and environmental profit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(a) of KoreanPatent Application No. 10-2007-0086677, filed on Aug. 28, 2007, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a multi-layered polymer electrolytemembrane for a fuel cell and a fuel cell comprising the membrane. Moreparticularly, the present invention relates to a multi-layered polymerelectrolyte membrane comprising an anion binding substance coated on thesurface of a conventional non-aqueous polymer electrolyte membrane, anda fuel cell comprising the membrane, which can prevent elution of acidand improve electrochemical stability of the fuel cell.

BACKGROUND ART

In the modem society to today, main sources of energy have been fossilfuel, nuclear energy and waterpower. However, as these energy sourcesare being exhausted and may cause environmental problems, many countrieshave attempted develop an alternative energy.

Recently, as the role of an alternative energy increases due to thedrastic rise in oil price and more strict environmental regulations byUNFCC (the United Nations Framework Convention on Climate Change), afuel cell has been spotlighted as the next-generation energy source.

The fuel cell is a device that can convert chemical energy of a fuel toelectric energy. Unlike others, the fuel cell is not restricted byCarnot cycle, thus showing a remarkably high efficiency and generatesrelatively less noise, vibration and waste gas. The fuel cell may alsogenerate electric energy continuously as long as fuel and oxidant aresupplied. Depending on the kind of electrolyte and the operationtemperature, it may be divided into alkali fuel cell (AFC), phosphoricacid fuel cell (PAFC), molten carbonate fuel cell (MCFC), polymerelectrolyte membrane fuel cell (PEMFC), solid oxide fuel cell (SOFC),direct methanol fuel cell (DMFC), etc.

In particular, polymer electrolyte membrane fuel cell (PEMFC) shows arapid start-up due to a low operation temperature and is easy tomanufacture using a solid electrolyte. In addition, it has superioroutput density and energy conversion efficiency. For these reasons, ithas been intensively studied for portable, home and military uses or asan electric source of a car or an energy source for distributedgeneration.

FIG. 1 shows the principle of a polymer electrolyte membrane fuel cell(PEMFC). Protons produced when hydrogen is oxidized in an anode reactwith oxygen in a cathode, thus generating water and electricity.

Currently most popular fuel cell polymer electrolyte membrane is Nafion,which is a perfluorosulfonic acid based polymer. Nafion, however, hasserious drawbacks of high price and deterioration of cell performance ata temperature of higher than 80° C. due to the decrease in protonconductivity caused by dehydration of membrane. Therefore, PEMFC, whichincludes an aqueous system, shows a serious deterioration of electrodedeactivation due to a low operation temperature and the poisoning causedby carbon monoxide (CO). Further, this requires an additional watermanagement for humidifying the membrane, thus decreasing theproductivity of the fuel cell and preventing the commercialization ofthe fuel cell.

To overcome the aforementioned problems, there have been attempts madeto use materials that are superior in proton conductivity,electrochemical stability and thermal stability even under a hightemperature non-aqueous condition as a polymer electrolyte of a fuelcell. Among these attempts, Japanese patent application publication No.2000-195528 discloses a method of doping phosphoric acid inpolybenzimidazole-based polymer electrolyte. However, this method has aproblem that water produced on a cathode causes the elution ofphosphoric acid, resulting in decrease in the proton conductivity ofelectrolyte membrane.

Therefore, there is a need for a new non-aqueous polymer electrolytethat has a decreased price and improved high-temperature stability,salvation stability and/or electrochemical stability.

The above information disclosed in this Background Art section is onlyfor enhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

To overcome the aforementioned problems, the present inventors haveexerted extensive researches and finally reached the present invention.One aspect of the present invention is to provide a non-aqueous polymerelectrolyte membrane for a fuel cell, comprising an anion bindingsubstance selected from the group consisting of compounds of Formulas 1,2 and 3 as a coating layer:

[Formula 1]

BY

wherein Y is PO₄ or N;

[Formula 2]

BZ₃

wherein Z is Cl, I, Br, CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO,(CF₃)₂C(C₆H₅)O, (CF₃)₃CO,C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O,CF₃C₆H₄O, (CF₃)₂C₆H₃O, or C₆F₅; and

wherein R is CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO, (CF₃)₂C(C₆H₅)O,(CF₃)₃CO, C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O, CF₃C₆H₄O, (CF₃)₂C₆H₃Oor C₆F₅.

Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF DRAWINGS

The above and other features of the present invention will be describedwith reference to certain exemplary embodiments thereof illustrated theattached drawings in which:

FIG. 1 schematically illustrates the structure and the operationprinciple of the general polymer electrolyte fuel cell (PEMFC);

FIG. 2 schematically illustrates the structure and the operationprinciple of the anion binding substance herein;

FIG. 3 shows a multi-layered polymer electrolyte membrane herein havingcoating layer containing an anion binding substance;

FIG. 4(A) is a process of preparing the conventional non-aqueous polymerelectrolyte membrane, and FIG. 4(B) is a process of preparing amulti-layered polymer electrolyte membrane with an anion bindingsubstance coated layer according to the present invention; and

FIG. 5 shows the time dependency of phosphoric acid elution of a fuelcells comprising the polymer electrolyte membranes prepared in Example 1and Comparative Example 1, respectively.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiment of thepresent invention, examples of which are illustrated in the drawingsattached hereinafter. The embodiments are described below so as toexplain the present invention by referring to the figures.

As discussed above, one aspect of the present invention provides anon-aqueous polymer electrolyte membrane for a fuel cell, comprising ananion binding substance as a coating layer on a non-aqueous polymerelectrolyte membrane, thereby preventing the problem of acid elution.

As shown in FIG. 2, an anion binding substance generates anions duringthe dissociation of an acid, the movement of the generated anions isrestricted by the ion-dipole binding with anion binding agent, therebyinhibiting the elution on a cathode caused by water.

As shown in FIG. 3, the present invention prevents the elution of acidgenerated on a cathode of a fuel cell by introducing the anion bindingsubstance as a coating layer, thereby providing a fuel cell having along-term stability and economic and environmental profits.

To overcome the problem associated with the conventional non-aqueouspolymer electrolyte membranes, i.e., insufficient durability due to theelution of acid, the present invention introduces an anion bindingsubstance of Formula 1, 2 or 3 as a coating layer to the non-aqueouspolymer electrolyte membrane.

[Formula 1]

BY

wherein Y is PO₄ or N.

[Formula 2]

BZ₃

wherein Z is halogen, aliphatic organic compound or aromatic organiccompound such as Cl, I, Br, CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO,(CF₃)₂C(C₆H₅)O, (CF₃)₃CO, C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O,CF₃C₆H₄O, (CF₃)₂C₆H₃O, or C₆F₅.

wherein R is aliphatic organic compound or aromatic organic compoundsuch as CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO, (CF₃)₂C(C₆H₅)O, (CF₃)₃CO,C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O, CF₃C₆H₄O, (CF₃)₂C₆H₃O or C₆F₅.

[Formula 1]

BY

wherein Y is PO₄ or N.

[Formula 2]

BZ₃

wherein Z is halogen, aliphatic organic compound or aromatic organiccompound such as Cl, I, Br, CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO,(CF₃)₂C(C₆H₅)O, (CF₃)₃CO, C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O,CF₃C₆H₄O, (CF₃)₂C₆H₃O, or C₆F₅.

wherein R is aliphatic organic compound or aromatic organic compoundsuch as CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO, (CF₃)₂C(C₆H₅)O, (CF₃)₃CO,C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O, CF₃C₆H₄O, (CF₃)₂C₆H₃O or C₆F₅.

In a preferred embodiment, the coating layer is prepared by admixing asmall amount of polymer matrix and an anion binding substance tomaintain the shape of the coating layer. The polymer matrix is ahomo-polymer or a co-polymer selected from the group consisting ofbenzimidazole, benzothiazole, benzooxazole, an imide compound, acarbonate compound, and a blend thereof, wherein the homopolymer is PBIwith benzimidazole or PBO with benzooxazole or PI with imide or PC withcarbonate, etc., while the copolymer is PBI-co-PI or PBO-co-PI, etc.Further, the coating layer comprises, preferably, 1-95 wt % of an anionbinding substance of Formula 1, 2 or 3 and 5-99 wt % of a polymer matrixrelative to the total weight of the coating layer.

When the amount of the anion binding substance is less than 1 wt %, theeffect of the anion binding substance is not sufficient, and it may alsobe difficult to apply the multi-layered membrane to proton polymerelectrolyte membrane due to relatively low proton conductivity. Bycontrast, when the amount is higher than 95 wt %, the coating layer maynot be formed. Most preferably, the anion binding substance and polymermatrix are used in the amount of 60 wt % and 40 wt %, respectively.However, the present invention is not limited to the aforementionedrange.

Preferable thickness of the coating layer is 1-10 μm. When the thicknessis less than 1 μm, the anion binding effect may not sufficient. When thethickness is higher than 10 μm, however, the proton conductivity maydecrease with the increase in resistance, thereby lowering theperformance of the cell.

A non-aqueous polymer electrolyte membrane according to the presentinvention comprises coating layer of a polymer electrolyte membrane fora fuel cell on a side or both sides of the non-aqueous polymerelectrolyte membrane.

Further, non-aqueous polymer electrolyte membrane used in the presentinvention comprises 1-95 wt % of acid and 5-99 wt % of polymer matrix.The polymer matrix of the non-aqueous polymer electrolyte membrane is ahomo-polymer or a co-polymer selected from the group consisting ofbenzimidazole, benzothiazole, benzooxazole, an imide compound, acarbonate compound, and a blend thereof, wherein the homopolymer is PBIwith benzimidazole or PBO with benzooxazole or PI with imide or PC withcarbonate, etc., while the copolymer is PBI-co-PI or PBO-co-PI, etc. Theacid is at least one selected among phosphoric acid, acetic acid, nitricacid, sulfuric acid, formic acid, a derivative thereof and a mixturethereof.

EXAMPLES

The present invention is described more specifically by the followingExamples. Examples herein are meant only to illustrate the presentinvention, but they should not be construed as limiting the scope of theclaimed invention.

Examples 1-5 Preparation of Polymer Electrolyte Membrane ComprisingCoating Layer

A polymer electrolyte membrane was prepared by introducing a coatinglayer as illustrated in FIG. 4(B). A 3-neck reactor was purged withnitrogen, and polymerization was conducted at 200° C. by adding3,4-diamino benzoic acid monomer in a solvent, polyphosphoric acid in anamount of 5 wt % relative to the solvent.

The polymer was casted on glass plate using Doctor Blade, and placed atroom temperature for more than 36 hours. The polyphosphoric acid washydrolyzed by moisture into phosphoric acid.

A polymer electrolyte membrane for a fuel cell coated with anion bindingsubstance was prepared by introducing a coating layer having a thicknessof 3 μm, which comprises anion binding substance containing 60 wt % ofBPO₄ and 40 wt % of PBI as a polymer matrix to this polymer electrolytemembrane. Other polymer electrolyte membranes for a fuel cell were alsoprepared as shown in Table 1.

Comparative Examples 1-3

As illustrated in FIG. 4(A), non-aqueous polymer electrolyte membraneswithout a coating layer containing anion binding substance, which havethe same doping level, were prepared same as in Example 1 except thatthe coating layer was not coated.

TABLE 1 Thickness of Examples Anion binding substance Used amountcoating layer Ex. 1 BPO₄ 60 wt % 3 μm Ex. 2 BPO₄ 60 wt % 1 μm Ex. 3 BPO₄60 wt % 10 μm Ex. 4 BC₆H₅O 42 wt % 5 μm Ex. 5 Al(CF₃)₂C(C₆H₅)O 45 wt %10 μm Comp. Ex. 1 — 0 wt % 0 μm Comp. Ex. 2 — 0 wt % 1 μm Comp. Ex. 3 —0 wt % 10 μm

Experiment of Ion Conductivity and Doping Level Experimental Example 1

A cell was prepared by stacking polymer electrolyte membrane betweenTeflon electrodes prepared in Example 1. Resistance of electrolyte wasmeasured according to an AC impedance method. Ion conductivity anddoping level were obtained using the resistance, and the results werepresented in Table 2,

Experimental Examples 2-5

Ion conductivity and doping level were obtained same as in ExperimentalExample 1 except that the membranes prepared in Examples 2-5 were used,and the results are presented in Table 2.

Comparative Experimental Example 1

Ion conductivity and doping level were obtained same as in ExperimentalExample 1 except that the membranes prepared in Comparative Example 1was used, and the results are presented in Table 2.

Comparative Experimental Examples 2 and 3

Ion conductivity and doping level were obtained same as in ExperimentalExample 1 except that the membranes prepared in Comparative Examples 2-3were used, and the results are presented in Table 2.

TABLE 2 Examples Doping level Ion conductivity (150° C.) Exp. Ex. 1 29.3mol 1.3 × 10⁻¹ S/cm Exp. Ex. 2 29.2 mol 2.1 × 10⁻¹ S/cm Exp. Ex. 3 27.3mol 8.7 × 10⁻² S/cm Exp. Ex. 4 24.0 mol 6.4 × 10⁻² S/cm Exp. Ex. 5 27.2mol 7.9 × 10⁻² S/cm Comp. Exp. Ex. 1 28.3 mol 3.2 × 10⁻¹ S/cm Comp. Exp.Ex. 2 29.1 mol 4.5 × 10⁻¹ S/cm Comp. Exp. Ex. 3 27.5 mol 4.2 × 10⁻¹ S/cm

Measurement of Acid Elution of a Fuel Cell Prepared Using PolymerElectrolyte Membrane Comprising Coating Layer Experimental Example 4

The anion binding property of the polymer electrolyte membrane preparedin Example 1 was observed by immersing the membrane in 80 mL of waterfor 30 minutes at room temperature, followed by the measurement of theelution of acid according to the titration. The results are presented inFIG. 5.

As a result, when a polymer electrolyte membrane having a coating layerwas immersed in water for 10 minutes, 56% of acid remained and 44% ofacid was eluted into water. The elution of acid was inhibited by 45%,thus improving the durability of a non-aqueous polymer electrolytemembrane.

TABLE 3 Inhibition of Examples Remaining acid Elution acid acid elutionExp. Ex. 1 56% 44% 45% Exp. Ex. 2 47% 53% 34% Exp. Ex. 3 62% 38% 53%Comp. Exp. Ex. 1 20% 80% 0% Comp. Exp. Ex. 2 16% 84% −0.05% Comp. Exp.Ex. 3 21% 78% 0.3%

The inhibition of the acid elution was on the basis of the value of acidelution obtained in Comparative Experimental Example 1.

The elution of acid was remarkably reduced in Experimental Examples 2-3as compared to Comparative Experimental Examples 1-3. ExperimentalExamples show that the present invention may provide a fuel cell havingsuperior durability and a long-term stability.

In the present invention, a polymer electrolyte membrane was prepared bycoating anion binding substance onto the conventional non-aqueouspolymer electrolyte membrane, and a fuel cell was prepared by using themembrane, thereby preventing various problems such as the anion elutioncaused by water, the decrease in the proton conductivity and theinactivation of catalyst caused by the elution of acid.

Accordingly, the present invention may maintain the performance of afuel cell to the economic and environmental profit by overcoming theaforementioned problems.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A non-aqueous polymer electrolyte membrane for a fuel cell,comprising an anion binding substance selected from the group consistingof compounds of Formulas 1, 2 and 3 as a coating layer: [Formula 1] BYwherein Y is PO₄ or N; [Formula 2] BZ₃ wherein Z is Cl, I, Br, CH₃O,CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO, (CF₃)₂C(C₆H₅)O, (CF₃)₃CO,C₆H₅O, FC₆H₄O,F₂C₆H₃O, F₄C₆HO, C₆F₅O, CF₃C₆H₄O, (CF₃)₂C₆H₃O, or C₆F₅; and

wherein R is CH₃O, CF₃CH₂O, C₃F₇CH₂O, (CF₃)₂CHO, (CF₃)₂C(C₆H₅)O,(CF₃)₃CO, C₆H₅O, FC₆H₄O, F₂C₆H₃O, F₄C₆HO, C₆F₅O, CF₃C₆H₄O, (CF₃)₂C₆H₃Oor C₆F₅.
 2. The polymer electrolyte membrane of claim 1, wherein thecoating layer comprises 1-95 wt % of an anion binding substance ofFormula 1, 2 or 3 and 5-99 wt % of a polymer matrix.
 3. The polymerelectrolyte membrane of claim 2, wherein the polymer matrix is ahomo-polymer or a co-polymer selected from the group consisting ofbenzimidazole, benzothiazole, benzooxazole, an imide compound, acarbonate compound, and a blend thereof, wherein the homopolymerincludes PBI with benzimidazole or PBO with benzooxazole or PI withimide or PC with carbonate, while the copolymer includes PBI-co-PI orPBO-co-PI.
 4. The polymer electrolyte membrane of claim 1, wherein thecoating layer has a thickness of 1-10 μm.
 5. The polymer electrolytemembrane of claim 1, wherein the coating layer is coated on both sidesof the non-aqueous polymer electrolyte membrane.
 6. The polymerelectrolyte membrane of claim 1, wherein the non-aqueous polymerelectrolyte membrane comprises 1-95 wt % of an acid and 5-99 wt % of apolymer matrix.
 7. The polymer electrolyte membrane of claim 6, whereinthe polymer matrix is a homo-polymer or a co-polymer selected from thegroup consisting of benzimidazole, benzothiazole, benzooxazole, an imidecompound, a carbonate compound, and a blend thereof, wherein thehomopolymer includes PBI with benzimidazole or PBO with benzooxazole orPI with imide or PC with carbonate, while the copolymer includesPBI-co-PI or PBO-co-PI.
 8. The polymer electrolyte membrane of claim 6,wherein the acid is at least one selected from the group consisting ofphosphoric acid, acetic acid, nitric acid, sulfuric acid, formic acid, aderivative thereof and a mixture thereof.
 9. A fuel cell comprising thepolymer electrolyte membrane according to claim 1.